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The periodic table of the chemical elements (also periodic table
of the elements or just the periodic table) is a tabular display
of the chemical elements. Although precursors to this table exist,
its invention is generally credited to Russian chemist Dmitri Mendeleev
in 1869, who intended the table to illustrate recurring ("periodic")
trends in the properties of the elements. The layout of the table
has been refined and extended over time, as new elements have been
discovered, and new theoretical models have been developed to explain
The periodic table is now ubiquitous within the academic discipline
of chemistry, providing a useful framework to classify, systematize,
and compare all of the many different forms of chemical behavior.
The table has found many applications in chemistry, physics, biology,
and engineering, especially chemical engineering. The current standard
table contains 118 elements to date. (elements 1–118).
The layout of the periodic table demonstrates recurring ("periodic")
chemical properties. Elements are listed in order of increasing atomic
number (i.e., the number of protons in the atomic nucleus). Rows
are arranged so that elements with similar properties fall into the
same columns (groups or families). According to quantum mechanical
theories of electron configuration within atoms, each row (period)
in the table corresponded to the filling of a quantum shell of electrons.
There are progressively longer periods further down the table, grouping
the elements into s-, p-, d- and f-blocks to reflect their electron
In printed tables, each element is usually listed with its element
symbol and atomic number; many versions of the table also list the
element's atomic mass and other information, such as its abbreviated
electron configuration, electronegativity and most common valence
As of 2010, the table contains 118 chemical elements whose discoveries
have been confirmed. Ninety-four are found naturally on Earth, and
the rest are synthetic elements that have been produced artificially
in particle accelerators. Elements 43 (technetium), 61 (promethium)
and all elements greater than 83 (bismuth), beginning with 84 (polonium)
have no stable isotopes. The atomic mass of each of these element's
isotope having the longest half-life is typically reported on periodic
tables with parentheses. Isotopes of elements 43, 61, 93 (neptunium)
and 94 (plutonium), first discovered synthetically, have since been
discovered in trace amounts on Earth as products of natural radioactive
The primary determinant of an element's chemical properties is its
electron configuration, particularly the valence shell electrons.
For instance, any atoms with four valence electrons occupying p orbitals
will exhibit some similarity. The type of orbital in which the atom's
outermost electrons reside determines the "block" to which
it belongs. The number of valence shell electrons determines the
family, or group, to which the element belongs.
The total number of electron shells an
atom has determines the period to which
it belongs. Each shell is divided into
different subshells, which as atomic number
increases are filled in roughly this order
(the Aufbau principle) (see table). Hence
the structure of the table. Since the outermost
electrons determine chemical properties,
those with the same number of valence electrons
are grouped together.
Progressing through a group from lightest element to heaviest element,
the outer-shell electrons (those most readily accessible for participation
in chemical reactions) are all in the same type of orbital, with
a similar shape, but with increasingly higher energy and average
distance from the nucleus. For instance, the outer-shell (or "valence")
electrons of the first group, headed by hydrogen, all have one
electron in an s orbital. In hydrogen, that s orbital is in the
lowest possible energy state of any atom, the first-shell orbital
(and represented by hydrogen's position in the first period of
the table). In francium, the heaviest element of the group, the
outer-shell electron is in the seventh-shell orbital, significantly
further out on average from the nucleus than those electrons filling
all the shells below it in energy. As another example, both carbon
and lead have four electrons in their outer shell orbitals.
Note that as atomic number (i.e., charge on the atomic nucleus)
increases, this leads to greater spin-orbit coupling between the
nucleus and the electrons, reducing the validity of the quantum
mechanical orbital approximation model, which considers each atomic
orbital as a separate entity.
The elements ununtrium, ununquadium, ununpentium, etc. are elements
that have been discovered, but so far have not received a trivial